1. Field of the Invention
The present invention concerns a process and an apparatus for determining the content of total nitrogen in a sample by means of combustion elemental analysis.
2. Description of the Prior Art
The classical method for the determination of total nitrogen and proteins in a sample is the one known as wet Kjeldahl method. This method is based on the digestion of the sample with concentrated H.sub.2 SO.sub.4 and subsequent titration with ammonia. Though endowed with the outstanding advantage of enabling the analysis of samples in the order of 1-3 g, the Kjeldahl method has been recently more and more replaced by combustion methods, that allow a more simple process and its almost full automation.
The methods for combustion elemental analysis (or ultimate analysis) substantially provide to perform a flash dynamic combustion of small sample amounts in high excess of oxygen, in presence of catalysts, and the treatment of the thus obtained combustion gases by having them pass, under helium flow as carrier gas, successively through a copper-containing reduction reactor, a trap for CO.sub.2 absorption, a trap for water absorption and a gas chromatographic column packed with an adsorbent (e.g. Porapack QS) that has the function of separating nitrogen from other combustion gases. The determination of nitrogen eventually takes place by means of a thermal conductivity detector and the function of the Porapack column, besides the aforesaid one, is also that of improving the shape of the N.sub.2 peak and of providing the equipment with the necessary pneumatic resistance.
The major drawback of these methods is given by the fact that to obtain reliable results, only reduced amounts of samples, of about 10-20 mg, can be analyzed. Larger samples can be analyzed only if substantially deprived of organic compounds or with very reduced content thereof. This reduced capacity of analysis, as far as the sample quantity is concerned, leads to the need of homogenizing as accurately as possible (e.g. by grinding at liquid nitrogen temperature) the sample before drawing therefrom the portion to be analyzed, with all ensuing practical problems.
An attempt to solve said problems is given by the analytical method proposed by LECO company (USA). This method provides to perform a flash combustion of samples of 200-300 mg in O.sub.2 flow, and to treat a reduced amount, corresponding to about 10% or less, of the gases coming out from the reactor and previously diluted with He, in a way similar to the one above disclosed, The drawbacks of this method are due to the sealing problems deriving from the extreme complexity of the equipment, to the fact that in any case only part of the gases is analyzed, and to the problems of homogenization of the gases themselves.